ormolu: A formatter for Haskell source code

[ bsd3, development, formatting, library, program ] [ Propose Tags ]

A formatter for Haskell source code.

[Skip to Readme]


Manual Flags


Turn on development settings.


Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info


Maintainer's Corner

Package maintainers

For package maintainers and hackage trustees


  • No Candidates
Versions [RSS],,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Change log CHANGELOG.md
Dependencies ansi-terminal (>=0.10 && <1.0), array (>=0.5 && <0.6), base (>=4.14 && <5.0), binary (>=0.8 && <0.9), bytestring (>=0.2 && <0.12), Cabal-syntax (>=3.8 && <3.9), containers (>=0.5 && <0.7), Diff (>=0.4 && <1.0), directory (>=1.3 && <1.4), dlist (>=0.8 && <2.0), file-embed (>=0.0.15 && <0.1), filepath (>=1.2 && <1.5), ghc-lib-parser (>=9.4 && <9.5), gitrev (>=1.3 && <1.4), megaparsec (>=9.0), MemoTrie (>=0.6 && <0.7), mtl (>=2.0 && <3.0), optparse-applicative (>=0.14 && <0.18), ormolu, syb (>=0.7 && <0.8), text (>=0.2 && <3.0) [details]
License BSD-3-Clause
Maintainer Mark Karpov <mark.karpov@tweag.io>
Category Development, Formatting
Home page https://github.com/tweag/ormolu
Bug tracker https://github.com/tweag/ormolu/issues
Source repo head: git clone https://github.com/tweag/ormolu.git
Uploaded by mrkkrp at 2023-01-05T16:45:35Z
Distributions Arch:, Debian:, Fedora:, LTSHaskell:, NixOS:, Stackage:
Reverse Dependencies 2 direct, 1 indirect [details]
Executables ormolu
Downloads 24073 total (458 in the last 30 days)
Rating (no votes yet) [estimated by Bayesian average]
Your Rating
  • λ
  • λ
  • λ
Status Docs available [build log]
Last success reported on 2023-01-05 [all 1 reports]

Readme for ormolu-

[back to package description]


License BSD3 Hackage Stackage Nightly Stackage LTS Build status

Ormolu is a formatter for Haskell source code. The project was created with the following goals in mind:

  • Using GHC's own parser to avoid parsing problems caused by haskell-src-exts.
  • Let some whitespace be programmable. The layout of the input influences the layout choices in the output. This means that the choices between single-line/multi-line layouts in certain situations are made by the user, not by an algorithm. This makes the implementation simpler and leaves some control to the user while still guaranteeing that the formatted code is stylistically consistent.
  • Writing code in such a way so it's easy to modify and maintain.
  • Implementing one “true” formatting style which admits no configuration.
  • The formatting style aims to result in minimal diffs.
  • Choose a style compatible with modern dialects of Haskell. As new Haskell extensions enter broad use, we may change the style to accommodate them.
  • Idempotence: formatting already formatted code doesn't change it.
  • Be well-tested and robust so that the formatter can be used in large projects.

Try it out in your browser at https://ormolu-live.tweag.io! See Ormolu Live for more info.


The release page has binaries for Linux, macOS and Windows.

You can also install using cabal or stack:

$ cabal install ormolu
$ stack install ormolu

Ormolu is also included in several package repositories. E.g., on Arch Linux, one can use the package on AUR:

$ yay -S ormolu

Building from source

The easiest way to build the project is with Nix:

$ nix-build -A ormolu

Note that you will need to add IOHK Hydra binary cache, otherwise building may take a very long time.

Alternatively, stack could be used as follows:

$ stack build # to build
$ stack install # to install

To use Ormolu directly from GitHub with Nix, this snippet may come in handy:

  pkgs = import <nixpkgs> { };
  source = pkgs.fetchFromGitHub {
      owner = "tweag";
      repo = "ormolu";
      rev = "c1d8a8083cf1492545b8deed342c6399fe9873ea"; # update as necessary
      # do not forget to update the hash:
      sha256 = "sha256-3XxKuWqZnFa9s3mY7OBD+uEn/fGxPmC8jdevx7exy9o=";
in (import source {  }).ormoluExe # this is e.g. the executable derivation


The following will print the formatted output to the standard output.

$ ormolu Module.hs

Add --mode inplace to replace the contents of the input file with the formatted output.

$ ormolu --mode inplace Module.hs

Use find to format a tree recursively:

$ ormolu --mode inplace $(find . -name '*.hs')

Or find all files in a project with git ls-files:

$ ormolu --mode inplace $(git ls-files '*.hs')

To check if files are are already formatted (useful on CI):

$ ormolu --mode check $(find . -name '*.hs')

Beware git's core.autocrlf on Windows

Ormolu's output always uses LF line endings. In particular, ormolu --mode check will fail if its input is correctly formatted except that it has CRLF line endings. This situation can happen on Windows when checking out a git repository without having set core.autocrlf to false.

Ormolu Live

On every new commit to master, Ormolu Live is deployed to https://ormolu-live.tweag.io. Older versions are available at https://COMMITHASH--ormolu-live.netlify.app.

Editor integration

We know of the following editor integrations:

Haskell Language Server

Haskell Language Server has built-in support for using Ormolu as a formatter.

GitHub actions

ormolu-action is the recommended way to ensure that a project is formatted with Ormolu.

Language extensions, dependencies, and fixities

Ormolu automatically locates the Cabal file that corresponds to a given source code file. When input comes from stdin, one can pass --stdin-input-file which will give Ormolu the location of the Haskell source file that should be used as the starting point for searching for a suitable Cabal file. Cabal files are used to extract both default extensions and dependencies. Default extensions directly affect behavior of the GHC parser, while dependencies are used to figure out fixities of operators that appear in the source code. Fixities can also be overridden if .ormolu file is found next to the corresponding Cabal file, i.e. they should be siblings in the same directory.

Here is an example of .ormolu file:

infixr 9  .
infixr 5  ++
infixl 4  <$
infixl 1  >>, >>=
infixr 1  =<<
infixr 0  $, $!
infixl 4 <*>, <*, *>, <**>

It uses exactly the same syntax as usual Haskell fixity declarations to make it easier for Haskellers to edit and maintain.

Besides, all of the above-mentioned parameters can be controlled from the command line:

  • Language extensions can be specified with the -o or --ghc-opt flag.
  • Dependencies can be specified with the -p or --package flag.
  • Fixities can be specified with the -f or --fixity flag.

Searching for both .cabal and .ormolu files can be disabled by passing --no-cabal.

Magic comments

Ormolu understands two magic comments:




This allows us to disable formatting selectively for code between these markers or disable it for the entire file. To achieve the latter, just put {- ORMOLU_DISABLE -} at the very top. Note that for Ormolu to work the fragments where Ormolu is enabled must be parseable on their own. Because of that the magic comments cannot be placed arbitrarily, but rather must enclose independent top-level definitions.


One can ask Ormolu to format a region of input and leave the rest unformatted. This is accomplished by passing the --start-line and --end-line command line options. --start-line defaults to the beginning of the file, while --end-line defaults to the end.

Exit codes

Exit code Meaning
0 Success
1 General problem
2 CPP used (deprecated)
3 Parsing of original input failed
4 Parsing of formatted code failed
5 AST of original and formatted code differs
6 Formatting is not idempotent
7 Unrecognized GHC options
8 Cabal file parsing failed
9 Missing input file path when using stdin input and accounting for .cabal files
10 Parse error while parsing fixity overrides
100 In checking mode: unformatted files
101 Inplace mode does not work with stdin
102 Other issue (with multiple input files)


  • CPP support is experimental. CPP is virtually impossible to handle correctly, so we process them as a sort of unchangeable snippets. This works only in simple cases when CPP conditionals surround top-level declarations. See the CPP section in the design notes for a discussion of the dangers.
  • Input modules should be parsable by Haddock, which is a bit stricter criterion than just being valid Haskell modules.

Running on Hackage

It's possible to try Ormolu on arbitrary packages from Hackage. For that execute (from the root of the cloned repo):

$ nix-build -A hackage.<package>

Then inspect result/log.txt for possible problems. The derivation will also contain formatted .hs files for inspection and original inputs with .hs-original extension (those are with CPP dropped, exactly what is fed into Ormolu).

Forks and modifications

We know of the following actively maintained forks:

  • Fourmolu, which uses 4-space indentation and allows arbitrary configuration.





Copyright © 2018–present Tweag I/O